This invention relates in general to vertical sweep signal processing for a CRT in a video display and in particular to circuitry for generating a sawtooth current waveform in the deflection coils of a CRT for controlling scan and retrace of an electron beam in a video display.
Since the present invention finds particular utility in the vertical deflection circuit of a CRT in a video display, it will be particularly described with reference to its use in such apparatus, although it will be appreciated that its uses are by no means limited to such applications.
Deflection circuits utilized in television receivers, and in CRT video displays in general, synchronize the deflection signals used to control the sweep of the electron beam with synchronizing pulses recovered from the composite video signals received by the television receiver or video display. These circuits typically make use of the charge-discharge cycle of a capacitor in generating a sawtooth current waveform having a predetermined period and magnitude. The ramp of the sawtooth current waveform is generally developed from the discharge of a capacitor while the capacitor is recharged during the retrace period. It is by means of this sawtooth current waveform imposed upon the CRT's deflection coils that the electron beam scans and retraces over the faceplate of the CRT at the appropriate times.
Various approaches to deflection circuit design and, in particular, synchronization oscillator design to achieve synchronization of electron beam sweep with input synchronization pulses are disclosed in the prior art. Early attempts in this area utilized switching diodes in combination with a voltage source to alternately charge and discharge the capacitor. These efforts were followed by the incorporation of transistors in CRT sawtooth current waveform generation circuits. These components made the electron beam deflection circuits more reliable, more economical and, in general, better performing in terms of switching speeds and power consumption. These transistorized sawtooth deflection generators, however, suffered from characteristic operating limitations imposed on the current and the voltage of transistor deflection circuits. These performance limitations gave rise to the development of silicon controlled rectifier (SCR) circuits formed of a semi-conductor assembly capable of operating at higher currents than that of normal rectifier which has been modified to "block" in the forward direction until a small signal is applied to the control electrode, or gate. After the control signal is applied, the device conducts in the forward direction with a forward characteristic very similar to that of a normal silicon rectifier and continues to conduct even after the control signal has been removed. In this manner, a multivibrator combination may be made for providing a stable oscillator for synchronizing sweep signals with synchronization input signals. The SCR-type oscillator-deflection circuits were not without limitations, however. Among their performance limitations were instabilities, or drift, in the signal voltage levels required to initiate the transistion to a stable oscillating state. Another inherent operating limitation in SCR oscillators is the requirement for an outside source of high power signals to terminate the SCR's oscillatory state.
One approach to the design of an SCR sawtooth wave generator is disclosed in U.S. Pat. No. 4,122,363. Disclosed therein is a circuit arrangement for obtaining a periodic sawtooth current in a coil utilizing a controllable switching device, or SCR, whose control electrode is connected to a source of periodic drive pulses which render the switching device conductive during part of the sawtooth period. While taking advantage of the reliability and ease of firing of thyristors, this invention requires a synchronization drive input pulse of a uniform width during each deflection cycle in order to initiate retrace. A lack of uniformity in synchronization input drive pulse width from one cycle to another can result in improper operation of the deflection circuit.
A transistorized vertical sawtooth deflection generator is disclosed in U.S. Pat. No. 3,435,282. This system provides a transistorized, self-oscillating vertical deflection generator utilizing a single capacitor of small value. Two coupled transistors form a self-oscillating network for charging a capacitor in a manner such that charge is being replaced on the negative terminal of the capacitor during discharge which has the effect of prolonging its discharge time and linearizing its rate of discharge. By also connecting the capacitor to a source of rising voltage during the trace interval the discharge of the capacitor is thereby impeded thus making it possible to use a capacitor of relatively small value while still achieving a slow rate of discharge. This system, however, was not designed to synchronize a DC-coupled vertical drive circuit with synchronization drive input signals of varying voltage amplitude and pulse width.
Still another approach to the design of a sawtooth current waveform generator for a CRT deflection system is disclosed in U.S. Pat. No. 4,110,666. This circuit makes use of two SCR's in combination one of which is turned on by the input synchronization pulses with the other turned on by pulses applied just before the end of line scan. The first SCR provides for electron beam scan synchronization while the second SCR provides for proper retrace of the electron beam. This arrangement suffers from the aforementioned trigger pulse turn-on instabilities and drift while being limited in the synchronization input signal pulse width with which it can operate.
The present invention, however, overcomes these limitations by providing a self-oscillating transistorized circuit possessing a predictable triggering level and capable of generating a sawtooth waveform of a given period and current, or voltage, level from input synchronization signals of variable pulse width and voltage level.